Nanoparticle density as a key design parameter for convective heat transfer enhancement in variable-property nanofluids: A numerical investigation

This study presents a particle-focused numerical investigation of convective heat transfer in variable-property nanofluids flowing through a vertical circular tube under constant wall heat flux. A validated CFD code, employing the low-Reynolds-number k-ε turbulence model of Chien, is used to simulate turbulent flow for four nanoparticle types—CuO, Al₂O₃, Ag, and Cu-coated CNT—dispersed in water. The effects of particle volume fraction (0–3%), particle diameter (15–90 nm), and inlet temperature (10–60°C) are systematically analyzed. The results reveal that nanoparticle density—a particle property often overlooked in favor of thermal conductivity—plays a dominant role in heat transfer enhancement. The highest enhancement is observed for Cu-coated CNT particles, with a 23.6% increase in the heat transfer coefficient at 3% volume fraction and Tᵢ = 30°C, followed by Ag (21.3%), CuO (9.1%), and Al₂O₃ (7.3%). Increasing the volume fraction from 1% to 3% approximately doubles the enhancement for all nanofluids, while raising the inlet temperature from 10°C to 60°C further improves the relative heat transfer coefficient by up to 9% for CuO at φ = 3%. A sensitivity analysis confirms that a + 20% change in particle density yields a + 16.9% change in the relative heat transfer coefficient, surpassing the influence of thermal conductivity (+10.9%) and specific heat (+7.8%). The performance evaluation criterion (PEC) indicates that Cu-coated CNT nanofluid offers the best thermohydraulic performance (PEC = 1.119 at φ = 1% and 30°C), while Ag also maintains a favorable PEC above 1.085. Smaller particle diameters (15 nm) provide greater enhancement than larger ones (90 nm) due to increased interfacial area, with an additional 1.6% improvement in the heat transfer coefficient for CuO at φ = 1%. These findings establish nanoparticle density as a critical parameter in powder and particle design for advanced thermal management systems, providing new quantitative guidelines for selecting and engineering nanoparticles in heat transfer applications.